Because Jupyter originates from the IPython project, hence the ipynb (IPython NoteBook) extension.
IPython initially was a monolithic project that offered the notebook interface and the Python kernel in a single release.
When the notebook interface was moved to a separate project, I suppose they kept the file extension for backwards compatibility.
IPython initially was a monolithic project that offered the notebook interface and the Python kernel in a single release. When the notebook interface was moved to a separate project, I suppose they kept the file extension for backwards compatibility.